The present invention relates to a fault system for a group of repeaters forming a repeatered PCM transmission system.
A repeatered PCM transmission system is basically composed of unit repeatered transmission lines and two monitoring stations terminating both ends of a transmission line. The unit repeatered transmission line consists of a plurality of repeaters cascade-connected with transmission line-sections. These plural repeaters are located at appropriate intervals and installed generally into unattended manholes. The output signals from the circuits are transmitted to the transmission monitoring station via a communication line different from the unit repeatered transmission lines such as represented, for example, by an interstitial pair. In the above-described fault location system, the transmission monitoring station transmits location codes including a center frequency component of the band-pass filter for designating a particular repeater, and the position of fault and the state of fault are diagnosed by analyzing the signals containing the fault information which are extracted by the circuit for that exclusive purpose built into each repeater and which are then sent back therefrom.
In the repeatered PCM transmission system in general, if a fault occurs in the system, a reception monitoring station detects the fault in the unit repeatered transmission line and, the line is immediately switched to a stand-by repeatered transmission line to maintain normal system operation. Simultaneously, a transmission monitoring station commences a fault location process for locating where in the repeater group including the transmission line-sections the fault has arisen with respect to the faulty line switched off.
The fault location process is carried out through a remote control operation because a group of repeaters are installed at unattended points remote from the monitoring stations. Accordingly, each of a plurality of repeaters is provided with a circuit for the exclusive use of the fault location, and a band-pass filter whose center pass-band frequency is designed to be different from one repeater to another is included in the circuit.
As a typical example of the PCM repeater fault location systems, a built-in frequency divider (flip-flop circuit in the case of twin pulse use to be described later in detail) system employing the fault location codes basically consisting of a plurality of periodic twin pulse groups as disclosed in FIG. 3 of an article titled "PCM-400M Digital Repeatered Line" on pp. 24-11 - 24-15 of "Conference Record," Vol. 3 published in INTERNATIONAL CONFERENCE ON COMMUNICATIONS, San Francisco, California, June 16-18, 1975 (Reference 1) has been proposed heretofore.
This frequency divider system with the above-mentioned circuit can realize an excellent fault location system of high precision using the built-in frequency divider, so that it is capable of sensing even a small repeater transmission error, because the transmission error is sent back as a phase inversion or an amplitude attenuation of a repeater-designation frequency component extracted by the circuit. However, this system is not adaptable to a fault location system for use in a high speed repeater transmission system since a plurality of the periodic pulse groups must be employed as the fault location signal as shown in FIG. 3(A) in Reference 1. More particularly, as is well-known, the repeater transmission errors in a high speed system are mainly caused by the intersymbol-interference caused between the transmitted codes, and the rate of the error is largely affected by the construction of the codes to be repeated and transmitted. For instance, in the case of using a plurality of the periodic pulse groups as the fault location codes, the error rate obtained by the use of the location codes is greatly different from that observed in practical repeatered transmission signals of a random nature because the fault location codes are constructed using periodic codes having non-random nature.
As a result, the great difference between the error rates monitored in the practical transmission and obtained with the fault locating process in a high speed system makes a fault locating process impossible to perform the normal operation. This shortcoming in the prior art system is hereinafter called "off-line effect".